What about U.S. participation in a no-fly zone in Libya based on humanitarian needs?
Doesn’t work. The legal basis for that no-fly zone was U.N. Security Council Resolution 1973, adopted March 17, 2011, which imposed “a ban on all flights in the airspace of the Libyan Arab Jamahiriya in order to help protect civilians.”
The Security Council also authorized member states who have notified the Security Council and League of Arab States, acting nationally or through regional organizations, “to take all necessary measures to enforce compliance.”
The reference to “all necessary measures” is “the standard phrase the Security Council uses to authorize states to act militarily,” according to a Yale Journal of International Law Online article by Michael N. Schmitt, a former professor of international law at the U.S. Naval War College.
There’s no such resolution for Syria. Without one, or without Congress approving a Syrian resolution, there is no legal basis for U.S. forces striking Syrian targets.